The present invention relates to restoring and clocking pulse width modulated data in a data recording channel during either writing or reading of the data.
Pulse width modulated data is a form of data where transitions in the representation of data on a storage medium correspond to a bit of data, either a "1" or a "0". No transition thus corresponds to a "0" or a "1" respectively. Since the condition of no transition carries information, it is necessary to use a clock signal to define data cell width in order to detect how many no transitions have occurred between transitions. This clock signal must be synchronized with the data-in signal, and this is usually done with a phase-lock loop. A good example is taught in U.S. Pat. No. 3,804,992 entitled "Digital Time Sampling Phase Comparator With Noise Rejection," issued to Fiorino et al.
In a typical environment, magnetic recording for example, the pulse width modulated recordings tend to be uniform, i.e. have similar precision and definition throughout a recording. The data transitions tend to shift in the same direction and in small amounts relative to the clock and thus the phase-lock loop can control a clock to track the time shifts of the data-in signal.
In optical storage systems, many factors lead to variations in the size of the representation of data in a single recording whether the representation is the leading and trailing edges of a pit which reflects light, or the size of the area of changed magnetic or opto-magnetic properties. As a result, the transitions in pulse width modulated data may move a significant amount due to recording properties. This recording shift has nothing to do with the information content of the data.
The recording shift of the leading and trailing edge data transitions is in opposite directions. In effect the pulse width has changed due to variations in recording phenomenon rather than due to changes in timing or information. These unintended pulse width variations make it difficult for the phase-lock loop to correctly synchronize the clock to the data and thus align timing windows, i.e. data cells, used in a recording channel to decode the data from the transitions.
Some of the potential factors causing the leading and trailing edges to shift significantly in opposite directions are variable laser power, focus error, tracking error, media composition, laser dwell time and media sensitivity. Variations in these factors lead to timing variations in both the leading edge of the pit or mark and the trailing edge of the pit. If laser power is too great, or the media is slightly more sensitive than expected, the leading edge of the pit may be encountered sooner than expected because the pit is larger than desired. Under the same condition of the media, the trailing edge of the pit may be encountered later than expected, thus leading to timing difficulties in detection of both the leading and trailing edges. Merely moving the timing windows (data cells) one direction or the other by adjusting the clock in the phase lock loop can not compensate for these unintended pulse width variations and can lead to a larger number of errors in detecting the data.